Method of producing a liquid crystal display apparatus having a random orientation alignment film

Abstract: A liquid crystal display apparatus includes: a pair of substrates held at a distance; a pair of electrodes formed on opposing inner faces of the substrates, respectively; alignment films formed on the opposing faces to cover the pair of electrodes, respectively, the alignment films having capability to align a liquid crystal in a horizontal orientation and capability to align the liquid crystal in a random orientation at a predetermined temperature or more; and a chiral nematic liquid crystal layer interposed between the pair of alignment film, the chiral nematic liquid crystal layer having a plurality of microscopic domains, each of the microscopic domains having the liquid crystal molecules aligned uniformly in the vicinity of the alignment films, the liquid crystal molecules in different microscopic regions being arranged in different directions with respect to one another.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a liquid crystal display apparatus, and more particularly, to a liquid crystal display apparatus having a wide viewing angle. 2. Description of the Related Art: As the screen size and capacity of the liquid crystal display apparatuses increases, vigorous efforts have been made to increase the use of liquid crystal display apparatuses, which utilize the electro-optic characteristics of a liquid crystal, in wider applications in the field of office automation equipment. Liquid crystal display apparatuses currently in common use are classified by their respective operation modes. Generally, there are two types; twisted nematic (TN) type wherein liquid crystal molecules interposed between two glass substrates show an orientation twisted 90°, and super twisted nematic (STN) type wherein the liquid crystal molecules show orientations twisted 180° to 270°. The TN type is used mainly in active matrix liquid crystal display apparatuses, and the STN type is used in simple matrix type liquid crystal display apparatuses. These apparatuses will be described below with reference to the accompanying drawings. In the case of the TN type used in active matrix type liquid crystal display apparatuses or in liquid crystal display apparatuses of small sizes, the liquid crystal molecules in the interface with the glass substrate are uniformly oriented in the same direction at a pre-tilt angle to the glass substrate, with the orientation being twisted 90° between the upper and lower glass substrates. The 90° twist in the orientation is obtained, in general, by rubbing rayon fabric or the like against an alignment film comprising a thin polyimide film formed on the glass substrate in one direction, and arranging the upper and lower substrates such that the directions on the two substrates are at right angles to each other. When a voltage is applied to a TN type liquid crystal display apparatus as shown in Figure 1, the liquid crystal molecules 101 in the state of orientation twisted 90° begin to respond. As the voltage increases beyond the threshold voltage, the state of twisted orientation decays and replaced by splay orientation, so that the major axes of the liquid crystal molecules 101 are caused to erect out of the plane of the glass substrates 102 and 103. When the liquid crystal molecules 101 are observed in a direction of angle from the normal line (Z axis) of the substrate while changing the azimuth , the major axes of the liquid crystal molecules 101 are not uniform in the direction of azimuth angle. As a result, apparent anisotropy of the index of refraction (Δn) of the liquid crystal molecules 101 changes with the azimuth angle, thereby changing the amount of birefringence (Δnd) which is the apparent anisotropy of the index of refraction multiplied by the thickness of the liquid crystal layer (d). Therefore, when polarizers are arranged on the outward surfaces of the upper and lower glass substrates 102 and 103 so that light absorbing axes 106 and 107 lie perpendicular to the rubbing directions 104, 105 and light is allowed to enter in the -Z axis direction, the intensity of transmitted light varies with the azimuth angle thereby causing asymmetry with respe t to the viewing angle. This asymmetry in the viewing angle poses problems. Particularly during display of half tones or gray scales, problems such as significant loss of contrast depending on the angle of view or deterioration in the picture quality such as reversal of the displayed image may occur. For this reason, a proposal to expand the viewing angle of a TN type liquid crystal display apparatus has been made by Toko, Kobayashi, et al. (Y. Toko, T. Sugiyama, K. Katoh, Y. Iimura, S. Kobayashi: SID 93 DIGEST, pp. 622-625 (1993)). According to the proposal, a liquid crystal cell is formed by injecting a chiral nematic liquid crystal into the cell while keeping a specified spacing without rubbing after forming the polyimide alignment film on a pair of electrodes. Switching of light by the application of voltage under parallel Nicol prisms or perpendicular Nicol prisms is made possible by setting the chiral pitch of the chiral nematic liquid crystal to four times the cell thickness. However, the liquid crystal display cell fabricated without rubbing proposed by Toko et al. requires the injection into the cell of a liquid crystal in the isotropic phase, which leads to such drawbacks as the necessity of new facilities to heat the entire liquid crystal cell above the isotropic phase temperature during injection, thereby making the proposal impractical. In the case where a liquid crystal cell without rubbing is fabricated by employing the method of injecting the liquid crystal in nematic phase, which has been employed in the manufacture of TN liquid crystal display elements, the liquid crystal is only partially oriented because of the liquid crystal flow during injection resulting in a drawback of lack in homogeneity which can be recognized by unaided eyes. Further, in such liquid crystal cells which have partially oriented due to the liquid crystal cell during injection, the inhomogeneity cannot be completely eliminated even after keeping the cell in the isotropic phase for several hours. Also in the case where a liquid crystal display cell is fabricated without rubbing, it is difficult to control the texture of the liquid crystal which is formed by cooling. Therefore, when observed in an oblique direction under crossed Nicol prisms, unevenness of the luminance sometimes becomes so large so as to cover a plurality of pixels, causing the problem of appearing rough to the eye. SUMMARY OF THE INVENTION European patent application No. 0 589 264, which is non prepublished prior art under Article 54(3) EPC discloses liquid crystal display with non-rubbed substrates. Liquid crystal is injected between a pair of substrates of a rubbing-free type. Liquid crystal molecules are oriented randomly in optional directions on the surface of the substrate. Macroscopically, liquid crystal molecules are oriented omnidirectionally in the substrate in-plane with the equal probability at each direction. Microscopically, liquid crystal molecules are generally omnidirectional. Types of orientation include a type wherein micro domains each oriented in one direction occupy the whole area of the substrate in-plane, a type wherein micro domains are dispersively distributed and domains successively changing the orientation occupy the areas among the micro domains, a type wherein liquid crystal molecules successively changing the orientation are present in a broad area, and other types. With the orientation being generally omnidirectional in the substrate in-plane, it is possible to form a liquid crystal display element in combination with a pair of polarizers. Optical anisotropy in the in-plane and adverse effects of rubbing can be prevented, and the display exhibits wide and uniform viewing-angle characteristics. The present invention relates to a method of producing a liquid crystal apparatus comprising the steps of: forming a pair of electrodes on respective inner faces of a pair of substrates (3a, 3b); forming alignment films of an organic polymer (5a, 5b) on the substrates to cover the electrodes, alignment films having the capability to align the molecules of a liquid crystal material in a horizontal or slightly pre-tilted alignment without needing an orientation process such as rubbing; holding the substrates to oppose the alignment films at a predetermined distance; and injecting a liquid crystal material (1) having a chiral nematic phase in the space between the substrates; the method being characterised in that the organic polymer has a predetermined glass transition temperature, above which it exhibits an elasticity of rubber and; in that, after the injection of the liquid crystal materials, the method further comprises the step of: heating the apparatus at a temperature which is at least the highest one of said predetermined glass transition temperature and the nematic-isotropic transition temperature of the liquid crystal material; and cooling the device to the ambient temperature, whereby a horizontal or slightly pre-tilted alignment is induced in the liquid crystal molecules with the plurality of microscopic domains, in which the molecules exhibit a uniform alignment on the films, and which orientation is-randomly different from domain to domain. Preferably the organic polymer is a partially crystalline polymer. According to another embodiment the organic polymer is a cross-linked polymer. According to still another embodiment the organic polymer is a polyurethane type polymer or includes a polyurethane type polymer as an ingredient. In a preferred embodiment the chiral nematic liquid crystal layer has a spontaneous helical pitch p, and p satisfies a following inequality: 0.25≤d/p < 1 where d is the thickness of the chiral nematic liquid crystal layer. The liquid crystal display apparatus of the invention is made by forming an alignment film, having the property of loosing the force which regulates the liquid crystal orientation above a particular temperature, on an electrode. With such a liquid crystal display apparatus, disturbance in the orientation due to the flow caused when the liquid crystal is injected between two electrodes at the room temperature can be eliminated by heating the same. It is assumed that the disturbance in the orientation due to the liquid crystal flow is caused by the fact that the liquid crystal is adsorbed to the alignment film surface in the direction of flow, rendering some regularity to the surface of the alignment film. This anisotropy in the alignment film surface, once it is generated, cannot be eliminated even when the liquid crystal is heated above the NI point (nematic-isotropic point) of the liquid crystal, and remains as a disturbance in the orientation. In the alignment film used in the liquid crystal display apparatus of the invention, however, molecular chains can move freely to a certain degree when heated above a particular temperature. And the liquid crystal material exhibits Brownian movement when the liquid crystal is heated above the NI point and changed into an isotropic liquid. When this happens, the anisotropy of the alignment film caused by the liquid crystal flow at the time the liquid crystal was injected between the electrodes is lost, resulting in random orientation of adjacent liquid crystal molecules. Consequently, the liquid crystal display apparatus of the invention does not include such unevenness that is observable with unaided eyes and is caused by disturbances in the orientation due to the flow. Also the liquid crystal display apparatus of the invention utilizes random orientation which the liquid crystal molecules spontaneously take when the liquid crystal is cooled from above the NI point down to below the NI point. When the random state is observed in an oblique direction below crossed Nicol prisms, coarse luminance distribution may result in a rough appearance of the display to the eye. This problem can be solved by forming regions of different surface conditions on the alignment film surface, in order to render discontinuity between the regions of different surface conditions to the randomly oriented structure, thereby reducing the coarse luminance distribution and alleviate the rough appearance of display. Thus, the invention described herein makes possible the advantages of (1) providing a liquid crystal display apparatus having a wide viewing angle without involving a significantly difficult process and (2) providing a liquid crystal display apparatus increasing the viewing angle without rough appearance to the eye. These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an explanatory view of the orientation of the liquid crystal in a twisted nematic type liquid crystal display apparatus. Figure 2 is a sectional view of a liquid crystal display apparatus of the first example. Figure 3 is a perspective view illustrating a polarizer-integrated type substrate which can be used in the liquid crystal display apparatus of the first example. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described by way of examples. Example 1 Figure 2 is a cross sectional view of a liquid crystal display apparatus 10 of the invention. The liquid crystal display apparatus 10 has substrates 3a and 3b which have transparent electrodes 2a and 2b on the opposing surfaces thereof, and a liquid crystal layer 1 interposed between the substrates 3a and 3b . The liquid crystal display apparatus 10 has polarizers 4a and 4b on surfaces of the substrates 3a and 3b which do not oppose each other. The liquid crystal display apparatus 10 of the invention changes its transmissivity to light, which passes from the polarizer 4a through the liquid crystal layer 1 to the polarizer 4b , depending on the magnitude of the electric field applied to the liquid crystal layer 1 through the electrodes 2a and 2b similarly to the case of a liquid crystal display apparatus 10 of the prior art. It is preferred that the polarizers 4a and 4b are arranged so that the axes of absorption of the two polarizers 4a and 4b lie parallel or perpendicular to each other. More preferably, the axes of the absorption of the two polarizers 4a and 4b lie perpendicular to each other. The liquid crystal display apparatus 10 of the invention has organic polymer films 5a and 5b having glass transition points formed on the transparent electrode 2 of the substrates 3a and 3b . The liquid crystal layer 1 includes a chiral nematic liquid crystal produced by adding a small amount of chirality agent to a nematic liquid crystal. For the nematic liquid crystal and the chirality agent, known materials used in twisted nematic (TN) liquid crystal display apparatuses and super-twisted nematic (STN) liquid crystal display apparatuses may be used. The chirality agent may be, for example, CB15, C15 made by BDH Co., Ltd., CN, R811, S811, R1011, S1011 made by E. Merck, CM-19, CM, CM-20, CM-21, CM-22 made by Chisso Co., Ltd. The ratio d/p of the thickness d of the liquid crystal layer 1 to the spontaneous helical pitch p of the chiral nematic liquid crystal is preferably from 0.1 to 1, and more preferably from 0.1 to 0.75, while 0.25 is the most preferable because it causes less coloration of the liquid crystal display apparatus. The substrates 3a and 3b may be any material, provided that it is transparent to visible light. For example, the substrates 3a and 3b may be a known transparent material such as glass, acrylic resin and polycarbonate resin. The transparent electrodes 2a and 2b of tin oxide or ITO (indium-tin oxide) are formed on the substrates 3a and 3b by means of deposition, sputtering, CVD or the like. Over these surfaces, the organic polymer films 5a and 5b having glass transition points are formed. While the organic polymer films 5a and 5b may be of any type, it preferably has a glass transition point in a range from 40°C up to 150°C. A glass transition point below 40°C is not preferable because the liquid crystal orientation changes while driving the liquid crystal display apparatus at room temperature. A glass transition point higher than 150°C, on the other hand, is not preferable because the liquid crystal, sealing resin or the like of the liquid crystal layer 1 experiences changes during the heating process at a temperature above the glass transition point for the purpose of eliminating disturbances in the orientation. While the organic polymer films 5a and 5b may be either amorphous, crystalline or partially crystalline polymers, provided that the films show an elasticity of rubber above the glass transition point. The elasticity of rubber referred to here means the ability to be stretched by at least 100% and return to almost the initial size when the external force is terminated. Moreover, the organic polymer films 5a and 5b should be a partially crystalline polymer or a crosslinked polymer in order to fix the liquid crystal orientation at a temperature below the glass transition point. However, it is apparent that such a resin, which dissolves into the liquid crystal, is significantly colored is not suitable for the organic polymer films 5a and 5b . For the organic polymer films 5a and 5b , polymethyl methacrylate, poly(t-butylmethacrylate), polycarbonate, polycyclohexylmethacrylate, polyethyleneterephthalate, materials based on polyurethane, poly(2-hydroxyethylmethacrylate), poly(isobornylmethacrylate), polyvinylalcohol and cellulose polymer, copolymers of these materials, or those which are partially crosslinked may be used, provided that the glass transition point is in a range from 40°C up to 150°C. Among the above-mentioned organic polymer films, it is preferable to employ materials based on polyurethane, and especially, partially crosslinked thermoplastic polyurethane. Organic polymer films used for the invention must have the capability to align the liquid crystal in horizontal orientation. For example, organic polymer films having an alkyl chain or a trifluoromethyl group in the side chain can be used even though they are known as a film having a high pre-tilt angle. On the other hand, a film made of polysiloxane is not suitable for an organic film of the invention because the film aligns the liquid crystal in a perpendicular orientation. Moreover, a mixture of the organic polymer mentioned above and materials known as a conventional alignment film may be used for the invention. In such a case, it is preferable that the mixture contains the above-mentioned organic polymer at 3% or more by weight. The organic polymer films 5a and 5b are formed by applying the organic polymer in the form of a solution onto the substrates 3a and 3b having the transparent electrodes 2a and 2b , then removing the solvent. Coating with the organic polymer may be carried out by a known method such as spin coating, printing or dipping. The organic polymer film having a glass transition point formed on the substrate must be heated to remove the solvent. While the heating temperature varies depending on the kind of solvent, heating on a hot plate at 100°C for one minute will suffice in the case in which N-methylpyrrolidone is used as the solvent. The thickness of the organic polymer films 5a and 5b is not limited, although the thickness is preferably within a range from 20 nm to 200 nm. A film thinner than 20 nm will not be capable of covering the entire surface uniformly, and a film thicker than 200 nm results in increased voltage drop by the film leading to poor display quality of the liquid crystal display apparatus. The organic polymer films 5a and 5b are formed on the surfaces of the substrates 3a and 3b whereon the transparent electrodes 2a and 2b are formed. There may be an insulator layer being formed between the electrode and the alignment film to prevent the upper and lower substrates from shortcircuiting. The insulator layer may be made of any material, while silicon oxide and silicon nitride have better insulation performance and transparency. The organic polymer films 5a and 5b which have been formed do not need an orientation process such as rubbing. The two substrates 3a and 3b , which have the organic polymer films 5a and 5b and the transparent electrodes 2a and 2b , are held to keep a proper space therebetween so that the organic polymer films 5a and 5b of the respective substrates oppose each other, and the liquid crystal layer 1 is interposed in the space. The liquid crystal layer 1 may be formed by a known method such as vacuum injection or liquid crystal dripping method. The two substrates are kept at a specified distance usually by arranging spherical particles made of glass or a synthetic resin between the substrates. It is preferable to bond the two substrates by means of an adhesive before or at the same time the liquid crystal is interposed therebetween. An epoxy resin is often used as the adhesive. After injecting the liquid crystal between the substrates 3a and 3b as described above, the substrates 3a and 3b interposing the liquid crystal therebetween are kept at the NI point (nematic-isotropic point) of the liquid crystal or the glass transition point (Tg) of the organic polymer films 5a and 5b , whichever is higher. While the duration of time of keeping it at the temperature is not limited, about two hours may be sufficient in the case where a hot air drier is used. It is not desirable to keep the liquid crystal cell at a high temperature for too long a time, because it may cause part of the liquid crystal to decompose. Because the organic polymer films 5a and 5b are not subjected to a rubbing treatment, the liquid crystal molecules adjacent to the organic polymer films 5a and 5b should be ideally aligned in a random direction having a predetermined pre-tilt angle. However, the liquid crystal molecules are actually adsorbed to the surface of the organic polymer films 5a and 5b in the direction into which the liquid crystal has been injected. The liquid crystal molecules are then aligned in random direction by keeping the liquid crystal cell at the NI point of the liquid crystal or the glass transition point of the organic polymer films 5a and 5b, whichever is higher.. The regularity of the surface of the organic polymer films 5a and 5b caused by injection of the liquid crystal also disappear. After cooling down the liquid crystal cell to the room temperature, the liquid crystal has a plurality of microscopic domains. Each of the plurality of the microscopic domains in the vicinity of the organic polymer films 5a and 5b has the liquid crystal molecules aligned uniformly, and the liquid crystal molecules in different microscopic regions are arranged in different directions with respect to one another. Accordingly, in the liquid crystal display apparatus of the invention, disturbance in the orientation of the liquid crystal molecules disappears. Specific embodiments of the invention will be described below. (Example 1-1) The transparent electrodes 2a and 2b made of ITO (indium-tin oxide) were formed on the entire surface of the substrates 3a and 3b having an area of 5 cm x 6 cm. A nitrobenzene solution of polyethyleneterephthalate (made by Scientific Polymer Products Corp.) having glass transition point at 81°C was applied onto the surfaces of the structure of the substrates 3a and 3b , having the transparent electrodes 2a and 2b , and dried thereby to form the organic polymer films 5a and 5b each 70 nm thick. The polyethyleneterephthalate in the form of a film 50 µm thick was stretched by 200% or greater at a temperature of glass transition point or higher, and showed the elasticity of rubber. This film also showed a peak in X-ray diffraction, indicating that it was partially crystallized. The substrates 3a and 3b were arranged so that the organic polymer films 5a and 5b oppose each other. After sealing the periphery of the substrates 3a and 3b with epoxy resin, except for one portion while keeping the substrates 3a and 3b at a distance of 7.5 µm, a nematic liquid crystal (product name ZLI2419 made by E. Merck) having an NI point at 64°C was sealed between the substrates by a vacuum injection process to form the liquid crystal cell having the liquid crystal layer 1. The liquid crystal injected in this process was one with a chirality agent (product name R811 made by E. Merck) added thereto to obtain a liquid crystal pitch of 30 µm. The liquid crystal cell was kept in a hot air drier at a temperature of 120°C for one hour, then allowed to cool down at room temperature. The liquid crystal cell was interposed between the two polarizers 4a and 4b which were arranged so that the polarization axes lie at right angles to each other, thereby to obtain the liquid crystal display apparatus 10 . When a square wave signal of 5 V at 60 Hz was applied to the liquid crystal display apparatus 10, light did not transmit through the electrodes, but did transmit through other portions. The contrast measured at pixels in a direction of 30° from the normal direction of the substrate 3a at a voltage of 5 V (60 Hz) showed a high value of 17/1. (Example 1-2) The transparent electrodes 2a and 2b made of ITO (indium-tin oxide) were formed on the entire surface of the substrates 3a and 3b having an area of 5 cm x 6 cm. Dimethylformamide solution of polycarbonate (made by Scientific Polymer Products Corp.) having a glass transition point at 150°C and a mean molecular weight of 64,000 was applied onto the surfaces of the structure of the substrates 3a and 3b having the transparent electrodes 2a and 2b , and dried thereby to form the organic polymer films 5a and 5b each 70 nm thick. The polycarbonate in the form of a film 50 µm thick was stretched by 200% or greater at a temperature of glass transition point or higher, and showed the elasticity of rubber. This film also showed a peak in X-ray diffraction, indicating that it is partially crystallized. The substrates 3a and 3b were arranged so that the organic polymer films 5a and 5b oppose each other. After sealing the periphery of the substrates 3a and 3b with epoxy resin, except for one portion while keeping the substrates 3a and 3b at a distance of 7.5 µm, a nematic liquid crystal (product name ZLI2419 made by E. Merck) having an NI point at 64°C was sealed between the substrates by a vacuum injection process to form the liquid crystal cell having the liquid crystal layer 1. The liquid crystal injected in this process was one with a chirality agent (product name R811 made by E. Merck) added thereto to obtain a liquid crystal pitch of 30 µm. The liquid crystal cell was kept in a hot air drier at a temperature of 155°C for one hour, then allowed to cool down at room temperature. The liquid crystal cell was interposed between the two polarizers 4a and 4b which were arranged so that the polarization axes lie at right angles to each other, thereby to obtain the liquid crystal display apparatus 10 . When a square wave signal of 5 V at 60 Hz was applied to the liquid crystal display apparatus 10 , light did not transmit through the electrodes, but did transmit through other portions. The contrast measured at pixels, in a direction of 30° from the normal direction of the substrate 3a at a voltage of 5 V (60 Hz), showed a high value of 18/1. (Example 1-3) The transparent electrodes 2a and 2b made of ITO (indium-tin oxide) were formed on the entire surface of the substrates 3a and 3b having an area of 5 cm x 6 cm. Dimethylformamide and an N-methylpyrrolidone solution of thermoplastic polyurethane resin (product name Diari MS5500 made by Mitsubishi Heavy Industries Ltd.) having a glass transition point at 55°C was applied onto the surfaces of the structure of the substrates 3a and 3b having the transparent electrodes 2a and 2b , and dried thereby to form the organic polymer films 5a and 5b each 70 nm thick. The thermoplastic polyurethane in the form of a film 50 µm thick was stretched by 400% or greater at a temperature at the glass transition point or higher, and showed the elasticity of rubber. This film also showed a peak in X-ray diffraction, indicating that it was partially crystallized. The substrates 3a and 3b were arranged so that the organic polymer films 5a and 5b oppose each other. After sealing the periphery of the substrates 3a and 3b with epoxy resin, except for one portion while keeping the substrates 3a and 3b at a distance of 7.5 µm, a nematic liquid crystal (product name ZLI2419 made by E. Merck) having an NI point at 64°C was sealed between the substrates by a vacuum injection process to form the liquid crystal cell having the liquid crystal layer 1 . The liquid crystal injected in this process was one with a chirality agent (product name R811 made by E. Merck) added thereto to obtain a liquid crystal pitch of 30 µm. The liquid crystal cell was kept in a hot air drier at a temperature of 80°C for one hour, then allowed to cool down at room temperature. The liquid crystal cell was interposed between the two polarizers 4a and 4b which were arranged so that the polarization axes lie at right angles to each other, thereby to obtain the liquid crystal display apparatus 10 . When a square wave signal of 5 V at 60 Hz was applied to the liquid crystal display apparatus 10, light did not transmit through the electrodes but did transmit through other portions. The contrast measured at pixels, in a direction of 30° from the normal direction of the substrate 3a at a voltage of 5 V (60 Hz), showed a high value of 19/1. (Example 1-4) The transparent electrodes 2a and 2b made of ITO (indium-tin oxide) were formed on the entire surface of the substrates 3a and 3b having an area of 5 cm x 6 cm . An N-methylpyrrolidone solution of thermoplastic polyurethane resin (product name Diari MS5500 made by Mitsubishi Heavy Industries Ltd.) having a glass transition point at 81°C and polyimide (product name AL5417 made by Japan Synthetic Rubber Co., Ltd.) was applied onto the surfaces of the structure of the substrates 3a and 3b having the transparent electrodes 2a and 2b , and dried thereby to form the organic polymer films 5a and 5b each 70 nm thick. The thermoplastic polyurethane is added to the polyimide by a weight ratio of 1:9. The substrates 3a and 3b were arranged so that the organic polymer films 5a and 5b oppose each other. After sealing the periphery of the substrates 3a and 3b with epoxy resin, except for one portion while keeping the substrates 3a and 3b at a distance of 7.5 µm, a nematic liquid crystal (product name ZLI2419 made by E. Merck) having an NI point at 64°C was sealed between the substrates by a vacuum injection process to form the liquid crystal cell having the liquid crystal layer 1. The liquid crystal injected in this process was one with a chirality agent (product name R811 made by E. Merck) added thereto to obtain a liquid crystal pitch of 30 µm. The liquid crystal cell was kept in a hot air drier at a temperature.of 80°C for one hour, then allowed to cool down at room temperature. The liquid crystal cell was interposed between the two polarizers 4a and 4b which were arranged so that the polarization axes lie at right angles to each other, thereby to obtain the liquid crystal display apparatus 10 . When a square wave signal of 5 V at 60 Hz was applied to the liquid crystal display apparatus 10, light did not transmit through the electrodes, but did transmit through other portions. The contrast measured at pixels, in a direction of 30° from the normal direction of the substrate 3a at a voltage of 5 V (60 Hz), showed a high value of 17/1. (Example 1-5) The transparent electrodes 2a and 2b made of ITO (indium-tin oxide) were formed on the entire surface of the substrates 3a and 3b having an area of 5 cm x 6 cm. An N-methylpyrrolidone solution of partially crosslinked urethane resin (product name Texin 192A made by Mobay Chemical Co., Ltd.) having glass transition point at 82°C was applied onto the surfaces of the structure of the substrates 3a and 3b having the transparent electrodes 2a and 2b , and dried thereby to form the organic polymer films 5a and 5b each 70 nm thick. The thermoplastic polyurethane in the form of a film 50 µm thick was stretched by 400% or greater at a temperature of glass transition point or higher, and showed the elasticity of rubber. This film also showed a peak in X-ray diffraction, indicating that it was partially crystallized. The substrates 3a and 3b were arranged so that the organic polymer films 5a and 5b oppose each other. After sealing the periphery of the substrates 3a and 3b with epoxy resin, except for one portion while keeping the substrates 3a and 3b at a distance of 7.5 µm, a nematic liquid crystal (product name ZLI2419 made by E. Merck) having an NI point at 64°C was sealed between the substrates by a vacuum injection process to form the liquid crystal cell having the liquid crystal layer 1. The liquid crystal injected in this process was one with a chirality agent (product name R811 made by E. Merck) added thereto to obtain a liquid crystal pitch of 30 µm. The liquid crystal cell was kept in a hot air drier at a temperature of 100°C for one hour, then allowed to cool down at room temperature. The liquid crystal cell was interposed between the two polarizers 4a and 4b , which were arranged so that the polarization axes lie at right angles to each other, thereby to obtain the liquid crystal display apparatus 10 . When a square wave signal of 5 V at 60 Hz was applied to the liquid crystal display apparatus 10 , light did not transmit through the electrodes, but did transmit through other portions. The contrast measured at pixels, in a direction of 30° from the normal direction of the substrate 3a at a voltage of 5 V (60 Hz), showed a high value of 18/1. (Example 1-6) A polarizer-integrated substrate was used instead of the glass substrates 3a and 3b of the embodiment 1-3. Specifically, as is shown in Figure 3, a polarizer-integrated substrate having a polarizer 13, polyethersulphon films 14 and 15 arranged to interpose the polarizer 13 , an undercoat layer 16 provided on the polyethersulphon films 14 and a transparent electrode (ITO) layer 17 formed on the undercoat layer 16 were used. For such a polarizer-integrated substrate, CST-7100 made by Sumitomo Bakelite Co., Ltd., for example, may be used. The polyethersulphon films 14 and 15 have optical isotropy. The polarizer 13 , the polyethersulphon films 14 and 15 , and the undercoat layer 16 correspond to the substrates 3a and the polarizer 4a , or the substrates 3b and the polarizer 4b shown in Figure 2. When such a substrate is used, the transparent electrode (ITO) layer 17 can be formed into a desired configuration to make transparent electrodes 2a and 2b . Similarly to the example 1-3, the organic polymer layer 5a or 5b , shown in Figure 2 , was installed to cover the undercoat layer 16 whereon the transparent electrode (ITO) layer 17 was partially formed, thereby to obtain a liquid crystal display apparatus. When a square wave signal of 5 V at 60 Hz was applied to the liquid crystal display apparatus, light did not transmit through the electrodes, but did transmit through other portions. (Comparative example 1-1) Instead of the organic polymer film used in the example 1-1, an N-methylpyrrolidone solution of polyimide varnish (product name LQ-S100 made by Hitachi Chemical Co., Ltd.) having glass transition point at 266°C was applied. After evaporating the solvent by means of a hot plate, the film was left to harden at 250°C for one hour, thereby forming a polyimide film 70 nm thick and making a liquid crystal display apparatus. When heated to 280°C, the film turned black and decomposed, therefore it was considered to have no elasticity of rubber. After injecting the liquid crystal, the liquid cell was not kept at a high temperature. When a square wave signal of 5 V at 60 Hz was applied to the liquid crystal display apparatus, light did not transmit through the electrodes but transmitted through other portions. However, streaks along the flow of the liquid crystal during injection whether the voltage was OFF or ON. Consequently, the liquid crystal was colored along the streak when the voltage was OFF, and uniformly black display was not obtained when the voltage was ON. (Comparative example 1-2) Instead of the organic polymer film used in the example 1-1, an N-methylpyrrolidone solution of polyimide varnish (product name LQ-S100 made by Hitachi Chemical Co., Ltd.) having a glass transition point at 266°C was applied. After evaporating the solvent by means of a hot plate, the film was left to harden at 250°C for one hour, thereby forming a polyimide film 70 nm thick and making a liquid crystal display apparatus. After injecting the liquid crystal, the liquid cell was kept in a hot air drier at a temperature of 80°C for one hour, then allowed to cool down at room temperature. When a square wave signal of 5 V at 60 Hz was applied to the liquid crystal display apparatus, light did not transmit through the electrodes, but did transmit through other portions. However, streaks along the flow of the liquid crystal during injection whether the voltage was OFF or ON. Consequently, uniformly black display was not obtained when the voltage was ON. (Comparative example 1-3) Instead of the organic polymer film used in the example 1-1, an N-methylpyrrolidone solution of polyimide varnish (product name LQ-S100 made by Hitachi Chemical Co., Ltd.) having a glass transition point at 266°C was applied. After evaporating the solvent by means of a hot plate, the film was left to harden at 250°C for one hour, thereby forming a polyimide film 70 nm thick and a liquid crystal display apparatus was made. After injecting the liquid crystal, the liquid cell was kept in a hot air drier at a temperature of 280°C for one hour, and the liquid crystal cell broke with the liquid crystal scattering to the outside, unable to fabricate a liquid cell. The liquid crystal cells obtained in the embodiments 1-1 through 1-5 and comparative examples 1-1 and 1-2 were compared by studying the orientation of each liquid crystal. Orientation was checked to see whether orientation defect is found or not by visual observation with no voltage being applied. Results are shown in Table 1. The liquid crystal display apparatus of the invention made by using an organic polymer film having a glass transition point in a range from 40°C up to 150°C or an organic polymer film which shows the elasticity of rubber at temperatures above the glass transition point has good liquid orientation and shows no non-uniformity in the liquid crystal orientation in the direction of flow which may be caused by liquid crystal injection. Glass transition point(°C) Rubber elasticity Appearance of display Example 1-1 81 Yes Uniform Example 1-2 150 Yes Uniform Example 1-3 55 Yes Uniform Example 1-4 No Uniform Example 1-5 82 Yes Uniform Comparative Example 1-1 266 No Remarkable disturbance Comparative Example 1-2 266 No Disturbance Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.

Claims: A method of producing a liquid crystal apparatus comprising the steps of: forming a pair of electrodes on respective inner faces of a pair of substrates (3a, 3b); forming alignment films of an organic polymer (5a, 5b) on the substrates to cover the electrodes, alignment films having the capability to align the molecules of a liquid crystal material in a horizontal or slightly pre-tilted alignment without needing an orientation process such as rubbing; holding the substrates to oppose the alignment films at a predetermined distance; and injecting a liquid crystal material (1) having a chiral nematic phase in the space between the substrates; the method being characterised in that the organic polymer has a predetermined glass transition temperature, above which it exhibits an elasticity of rubber and; in that, after the injection of the liquid crystal materials, the method further comprises the step of: heating the apparatus at a temperature which is at least the highest one of said predetermined glass transition temperature and the nematic-isotropic transition temperature of the liquid crystal material; and cooling the device to the ambient temperature, whereby a horizontal or slightly pre-tilted alignment is induced in the liquid crystal molecules with the plurality of microscopic domains, in which the molecules exhibit a uniform alignment on the films, and which orientation is randomly different from domain to domain. A method according to claim 1, wherein the organic polymer is a partially crystalline polymer. A method according to claim 1, wherein the organic polymer is a cross-linked polymer. A method according to claim 1, wherein the organic polymer is a polyurethane type polymer or includes a polyurethane type polymer as an ingredient. A method according to one of the preceding claims, wherein the chiral nematic liquid crystal layer has a spontaneous helical pitch p, and p satisfies a following inequality: 0.25≤d/p<1 where d is the thickness of the chiral nematic liquid crystal layer.

Inventors: IWAI YOSHIO, KOSAKO SHINYA, KURAI HISAKO, MIZUNO HIROAKI, MOCHIZUKI HIDEAKI, SATO SHIGEHIRO, IWAI, YOSHIO, KOSAKO, SHINYA, KURAI, HISAKO, MIZUNO, HIROAKI, MOCHIZUKI, HIDEAKI, SATO, SHIGEHIRO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: G02F1
IPC codes: G02F1